This morning, a second explosion occurred at a stricken nuclear power plant in Japan, blowing the roof off of a containment building, but not damaging the reactor. The blast is the latest development in Japan’s escalating nuclear crisis due to problems caused by the earthquake and tsunami. Friday morning, a first plant was reported to have a failed cooling system, and over the weekend, three more nuclear power plants experienced continuing problems.

Radioactive releases will most likely continue for months, but officials say that the risk to the public is low, despite levels of radiation outside the plant being twice what Japan considers to be safe. Still, experts say that the situation is much more similar to Three Mile Island than Chernobyl. During the 1986 Chernobyl disaster, an entire reactor blew up and vaporized its radioactive fuel, sending immense amounts of radiation into the atmosphere and resulting in an epidemic of thyroid cancer and increases in leukemia. The amount of particles released was a million times more than that at Three Mile Island, where a partial meltdown occurred, which is more similar to the issues Japan’s plants are experiencing.

The explosion this morning occurred in Unit 3 of the Fukushima Dai-ichi Nuclear Power Station. The same plant experienced a hydrogen blast in Unit 1 on Saturday, and operators have lost the ability to cool three of the plant’s reactors. They resorted to flooding the reactors with seawater in an attempt to cool them, a move many experts have called a “Hail Mary pass.” The cooling systems have also failed at three reactors in a plant not far from Dai-ichi. Despite pumping seawater in, water levels around the cores are still well below where they should be.

It is unclear if this morning’s explosion released radiation, but Saturday’s most definitely did. On Sunday, the Pentagon reported that helicopters flying 60 miles away from the Dai-ichi plant picked up small amounts of radioactive material, which suggests widespread environmental contamination.

As part of the emergency cooling process, reactor operators must periodically release plumes of radioactive steam to reduce pressure. The process may continue for a year or more even after fission has stopped, meaning that thousands of Japanese may not be able to return to their homes for a long time. To make up for power lost from the plants, Japan has implemented a series of rolling three-hour long blackouts.